You can't compare Pihu's situation to Rachna, though- she is clearly much older than Rachna and Mayank, and closer in age to Sangeeta. Seema, Sangeeta, and Pihu clearly have similar personalities, so she was probably always able to talk to them and converse about the same kinds of things. And judging by Dolu's age, she was probably also there when Sangeeta got married, so she was in the house nearly as an adult before Sangeeta came.
Rachna, on the other hand, is the youngest girl in the house. She's already quiet, but with two outspoken aunts, an outspoken sister, a domineering father, an outgoing cousin, and two uncles telling her what to do all the time, she is always afraid something is going to go wrong.
I know Gunjan's going to change her eventually, but considering she's got Chayya as a friend, I wonder why she hasn't picked up any of her personality traits at all. Although it is true- opposites attract.